1. Field of the Invention
The present invention is related to apparatus for measuring thermal properties of samples of materials.
2. Background of the Invention
A simultaneous thermal or differential thermal analyzer (SDT) comprises a combination of a thermogravimetric analyzer, TGA (also known as a thermobalance), and either a differential thermal analyzer (DTA) or a differential scanning calorimeter (DSC). Thus, the instrument allows a user to simultaneously measure mass changes and to monitor a signal based on sensible or latent heat changes in the sample.
Thus, an SDT allows a user to measure both the heat flows (DSC or DTA) and weight changes (TGA) associated with transitions in a material as a function of temperature and time in a controlled atmosphere. Simultaneous measurement of these key material properties not only improves productivity but also simplifies interpretation of results. The complementary information obtained allows differentiation between endothermic and exothermic events which have no associated weight loss (e.g. melting and crystallization) and those which involve a weight loss (e.g. degradation). The combined evaluation also assures identical experimental and sampling conditions for both measurements, thereby eliminating those sources of uncertainty. Simultaneous DSC-TGA covers a wide temperature range from below ambient to above 1500° C., making it a powerful tool for studying a wide variety of materials including organic materials, notably polymers, and ceramics, metals, and other inorganics.
Typically, the design of such an SDT instrument comprises a combination of an existing microbalance component with a DTA or DSC measuring component. In fact, early SDT instruments were based on existing laboratory balances.
Generally, there are two types of microbalances in common use in SDT and TGA instruments, both of which employ the null balance principle, in which a restoring force is applied to the balance structure to maintain the balance in equilibrium. The restoring force, which is proportional to the change in weight, is the measured quantity in each type of microbalance. In both cases, the restoring force is applied electromagnetically as a response to a displacement of the balance structure, which is typically detected by optical means. Using such a balance, a very high degree of mass sensitivity and a very high resolution of changes in mass are readily obtained.
A first type of balance is the dual arm meter movement balance, in which a d'Arsonval meter movement (also referred to herein as a “meter movement balance”) supports the balance beam and applies the restoring force as a torque.
In an SDT instrument that employs a d'Arsonval meter (also termed “meter movement”), when the sample weight in a sample holder connected to the balance changes during an experiment, a displacement sensor near the TGA balance senses movement of the balance away from the equilibrium position and electric circuitry generates the current necessary to restore the balance to equilibrium.
The second type of balance used in SDT and TGA instruments is the guided balance, in which the weighed mass is supported by a mechanism that constrains the movement of the weighed mass. Typically, the guided balance mechanism comprises a parallel four-bar linkage with elastic flexure pivots. This mechanism is termed a parallel guided balance. An electromagnetic actuator is used to apply a restoring force to the linkage, while a displacement sensor detects movement away from the equilibrium position and electric circuitry generates any current necessary to restore the balance to equilibrium.
SDT and TGA instruments may be classified as horizontal or vertical instruments based on the orientation of the heating furnace and the relative position of the balance. In principle, the measurement of weight may be perturbed by thermal expansion of the structure that extends into the furnace, and by forces exerted by movement of gas in the furnace caused by the action of purging the furnace or by buoyancy induced flows, in addition to buoyancy forces resulting from gas density changes. The magnitude of these weighing errors depends upon the configuration of the SDT instrument. In a horizontal furnace, thermal expansion of the beams that extend into the furnace may cause large weighing errors, while a vertical furnace configuration is largely immune to these effects, because thermal expansion occurs parallel to the Earth's gravitational field. On the other hand, vertical instruments are far more susceptible to fluid forces because thermal gradients in the furnace are parallel to the direction of the gravitational field which favors buoyancy driven flows and because the movement of the balance mechanism is parallel to the direction of purge gas flow. Horizontal instruments are largely immune to these forces because temperature gradients in the furnace are orthogonal to the gravitational field which is unfavorable to buoyancy driven flows and the movement of the balance mechanism is orthogonal to the direction of purge gas flow. Finally, buoyancy forces due to gas density changes may affect both horizontal and vertical furnace configurations to a similar degree.
As noted above, an SDT instrument combines a TGA measurement with a DTA or DSC type measurement, which requires that at least the sample side of the heat flow rate sensing device be supported by the balance mechanism. During sample measurement, a sample can be heated or cooled to examine changes in the sample induced by changes in temperature. In the case of sample heating, the heating takes place while at least a portion of the member supporting the sample extends into a furnace used to heat the sample. As the sample is heated, mass changes in the sample cause the balance mechanism to deflect from equilibrium, such that the restoring force needed to maintain the equilibrium can be measured. At the same time, a thermal signal (either DTA or DSC) is transmitted using wires that extend from the sample region to the stationary or fixed part of the instrument, so that analysis of the material changes taking place can be performed based upon the thermal signals received from the sample. Thus, the wires that carry the DSC or DTA signals from the sample region must connect the moving part of the balance to the fixed part. These signal wires typically exert a parasitic force on the balance that constitutes a weighing error.
Several factors can lead to the result wherein the wires contribute to weighing errors in SDT measurements. In principle, the forces exerted by the wires on the balance need not result in weighing errors, as long as the response in the wires to a displacement is linearly elastic. In other words, if the forces the wires exert are strictly linearly proportional to the displacement of the wires and the proportionality constant does not change, weighing errors caused by the wires could be avoided. If the response of the wires is not linearly elastic, weighing errors will result. Because the wires are usually deformed during installation in the SDT apparatus, the wires will almost always exert some force on the balance regardless of the balance position or whether any motion is taking place. No force would be exerted by the wires on the balance only if they were in their undeformed position. Another problem that may arise is that the wires may relax over time, resulting in changes in the force exerted by the wires. Typically, the wires are bent to the required shape when they are installed in the SDT apparatus, such that the deformation of the wires is at least partially plastic in nature. Over time, some of the plastic strain relaxes, thus changing the force exerted by the wire in a static position, as well as the force resulting from a displacement of the balance. In principle, the wires can be annealed or stress relieved, but given that they are generally fine and easily bent, it is difficult to handle and install the wires without deforming them.
In addition to wires, pivot structures that are necessary to connect fixed parts of a balance to moving parts of the balance, or that connect two moving parts of a balance, can introduce forces that may influence the measurement of sample weight.
Of the two types of balance, the meter movement type, given its lower mass and lower stiffness, is more sensitive and has faster dynamic response. The guided balance is more robust and is immune to the thermal expansion effects described above when used in a horizontal configuration. The guided balance-type SDT instrument may be used in conjunction with either the vertical or horizontal furnace configuration. Generally, the meter movement balance is used with the horizontal furnace configuration.
In the horizontal configuration, the meter movement balance is typically employed in a differential weighing configuration in which two balances, a sample and a reference balance, are operated in parallel. One balance weighs the sample and its container, while the other balance weighs an empty container or an inert reference sample in the container. Subtracting the reference weight measurement from the sample weight measurement eliminates the weighing error due to thermal expansion of the weighing structure and the weighing error due to buoyancy forces acting on the apparatus. Sample buoyancy forces are still a potential source of error in the dual balance configuration. Since the dual balance configuration employs a horizontal furnace configuration, the balances are isolated from forces arising from fluid motion, whether due to purge gas flow or to buoyancy differences resulting from temperature variations in the furnace, because these forces act orthogonally to the gravitational field.
In a dual balance meter movement type SDT (also termed “dual balance SDT” hereinafter) each of the sample and reference balances includes a meter movement component, optical displacement sensor and electronics to maintain the respective balance in the equilibrium position. Besides incurring undesirable cost because of duplication of components, a dual balance SDT system suffers from potential mismatches between the components of the two balance assemblies, such as in the meter movements. Another shortcoming of this design is that the meter movement components (or “meter movements”) must support the entire weight of the balance beam and DTA or DSC holder structure.
D'Arsonval meter movements may be made with either jewel bearings or a thin taut band supporting the rotating part of the meter. Generally, taut band suspensions are preferred because they operate without friction. Displacement of the moving part of the meter twists the taut band slightly. Elastic deflection of the taut band is very linear and highly repeatable, whereas friction in jewel bearings is far more nonlinear and far less repeatable. On the other hand, jewel bearing meter movements can support much larger loads than those supported by taut band suspensions. Because a taut band suspension must support the entire weight of the beam, the sample (or reference) holder, DTA or DSC sensor, and sample, the weighing capacity is limited to a small fraction of the capacity of the taut band, most of which is used to support the beam, holder, and sensor. Thus, the taut band instruments tend to have low weighing capacity.
In view of the above, it will be appreciated that further improvement of balance apparatus in SDT instruments is needed.